According to the US Environmental Protection Agency (EPA), human activities emit 4.9 million pounds (2,220 metric tons) of mercury compounds every year. Several studies have focused on the exposure and effects of mercury in predatory fish and marine mammals.
It is well-known that people should limit the consumption of certain types of fish, like tuna, due to mercury biomagnification. However, little is known about mercury accumulation in the brains of wild terrestrial species.
Presence of Neurotoxins in Mammalian Brain
Dr. Yulia Pushkar from Purdue University's College of Science has maintained a brain imaging program with her group, showing expertise in sample preparation and data analysis. Her research team was tasked with checking for mercury in the brains of mongooses collected on Okinawa Island in Japan.
At first, Pushkar was skeptical whether mercury could be detected since neurotoxic elements are present in ultra-low concentrations even if they get into the brain. Her team took the specimens to the Advanced Photon Source at Argonne National Laboratory, where the brains were exposed to intense X-rays. Surprisingly, brain scans revealed the presence of mercury in these invasive animals.
The mystery of how mercury enters the brain of mongoose remains unresolved. Experts blame the water they drink, the bird eggs they consume, exposure to minerals, or even the air they breathe. Regardless of the cause, scientists know this is a terrible sign.
According to Pushkar, mercury is very toxic at low concentrations since it can bind and affect the function of essential biomolecules. The impact of detoxification will depend on the uptake and binding constant inside the detected accumulations and potential leakage if brain cells die. Currently, there is no possible way to dissolve these aggregates safely from tissue, and there are no reports of reversing mercury poisoning of the neural system. The main approach that can be taken is to avoid any exposures.
After refining the scans, the researchers achieved a resolution of a few tens of nanometers to observe the affected cells of the brain. Their collaborative findings were discussed in the paper "High-resolution imaging of Hg/Se aggregates in the brain of small Indian mongoose, a wild terrestrial species: insights into intracellular Hg detoxification."
Filtration Mechanism of Mammals
As the researchers scanned brain samples, they traced areas of the brawl with higher mercury content. They identified that cells of the choroid plexus and astrocytes of the subventricular zone contain mercury-rich puncta (approximately 0.5 to 2 microns in size).
The team believes these cells help filter mercury from the blood and brain tissue. This mechanism is carried out with the help of selenium, although the particular selenium-containing biomolecules where mercury binds remains to be discovered.
Researchers expect the human brain to similarly react to mercury through interactions with choroid plexus cells and astrocytes. However, they cannot confirm yet if the human brain has enough selenium-containing biomolecules to bind to mercury.
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